Interferometers for measuring a stage position in an e-beam lithography tool are known. Paul. G. Harris et al. discloses an e-beam tool comprising an interferometer for measuring its stage position. The differential interferometer comprises a signal detection unit having a bi-refringent wedge for receiving a measurement beam and a reference beam that run in parallel. The wedge creates an angular deviation between the beams, and the beams then pass through a Polaroid aligned at 45° to make them interfere. The resultant interference pattern is a linear array of fringes. These fringes are aligned with an integrated linear photodiode array, called an electrograting. The electrograting produces four electrical signals of relative phase 0°, 90°, 180°, and 270°. The gain matching of the electrograting allows the differential amplifier to remove common mode intensity changes on the signals due to an intensity change of the laser, for example, which results in a good measurement performance. Despite being a differential interferometer, the interferometer is only capable of measuring a relative position of the stage. Furthermore, the interferometer is difficult to assemble, as the optical parts in the detector must be positioned and configured carefully.
It is an object of the present invention to provide a cost effective compact multi-axis differential interferometer capable of measuring a displacement and/or rotation between a reference surface and a measurement surface. It is a further object of the present invention to provide a differential interferometer, which is relatively easy to assemble.